Sulfiphilic FeP/rGO as a highly efficient sulfur host for propelling redox kinetics toward stable lithium-sulfur battery

Transition Metal Phosphides: The Rising Star of Lithium-Sulfur Battery Cathode Host

Lithium-sulfur batteries (LSBs) with ultra-high energy density (2600 W h kg-1 ) and readily available raw materials are emerging as a potential alternative device with low cost for lithium-ion batteries. However, the insulation of sulfur and the unavoidable shuttle effect leads to slow reaction kinetics of LSBs, which in turn cause various roadblocks including poor rate capability, inferior cycling stability, and low coulombic efficiency. The most effective way to solve the issues mentioned above is to rationally design and control the synthesis of the cathode host for LSBs. Transition metal phosphides (TMPs) with good electrical conductivity and dual adsorption-conversion capabilities for polysulfide (PS) are regarded as promising cathode hosts for new-generation LSBs. In this review, the main obstacles to commercializing the LSBs and the development processes of their cathode host are first elaborated. Then, the sulfur fixation principles, and synthesis methods of the TMPs are briefly summarized and the recent progress of TMPs in LSBs is reviewed in detail. Finally, a perspective on the future research directions of LSBs is provided.

A Freestanding 3D Skeleton with Gradationally Distributed Lithiophilic Sites for Realizing Stable Lithium Anodes

Lithium (Li) metal anodes are drawing considerable attention owing to their ultrahigh theoretical capacities and low electrochemical reduction potentials. However, their commercialization has been hampered by safety hazards induced by continuous dendrite growth. These issues can be alleviated using the ZnO-modified 3D carbon-based host containing carbon nanotubes (CNTs) and carbon felt (CF) fabricated by electroplating in the present study (denoted as ZnO/CNT@CF). The constructed skeleton has lithiophilic ZnO that is gradationally distributed along its thickness. The utilization of an inverted ZnO/CNT@CF-Li anode obtained by flipping over the carbon skeleton after Li electrodeposition is also reported herein. The synergistic effect of the Li metal and lithiophilic sites reduces the nucleation overpotential, thus inducing Li+ to preferentially deposit inside the porous carbon-based scaffold. The composite electrode compels Li to grow away from the separator, thereby significantly improving battery safety. A symmetric cell with the inverted ZnO/CNT@CF-Li electrode operates steadily for 700 cycles at 1 mA cm-2 and 1 mAh cm-2 . Moreover, the ZnO/CNT@CF-Li|S cell exhibits an initial areal capacity of 10.9 mAh cm-2 at a S loading of 10.4 mg cm-2 and maintains a capacity of 3.0 mAh cm-2 after 320 cycles.

Synthesis of the Ni2P-Co Mott-Schottky Junction as an Electrocatalyst to Boost Sulfur Conversion Kinetics and Application in Separator Modification in Li-S Batteries

To overcome the shuttling effect and sluggish conversion kinetics of polysulfides, a large number of catalysts have been designed for lithium-sulfur (Li-S) batteries. Herein, a Mott-Schottky junction catalyst composed of Co nanoparticles and Ni₂P was designed to improve polysulfide kinetics. Our investigations reveal the rearrangement of charges at the Schottky junction interface and the construction of the built-in electric field are crucial for lowering the activation energy of the dissolved Li₂S_n reduction and Li₂S nucleation reaction. Furthermore, a series of experimental and electrochemical tests were performed to demonstrate that the Schottky catalytic effect enhanced the synergistic catalytic effect. With a Ni₂P-Co@CNT catalyst, the battery exhibits an initial specific capacity of 874 mAh g^-1 at a rate of 4.0 C, and the decay rate per cycle is 0.049% in 700 cycles. Meanwhile, the battery shows 0.118% decay rate per cycle at 0.5 C in 100 cycles at a high sulfur loading of 10 mg cm^-2. The Schottky heterojunction structure proposed here has been shown to have a good catalytic effect on the reduction of Li₂S_n and nucleation of Li₂S, which provides a profound guidance for efficient and rational catalyst design.

Energy-Saving Synthesis of Functional CoS2/rGO Interlayer With Enhanced Conversion Kinetics for High-Performance Lithium-Sulfur Batteries

Lithium sulfur (Li-S) battery has exhibited great application potential in next-generation high-density secondary battery systems due to their excellent energy density and high specific capacity. However, the practical industrialization of Li-S battery is still affected by the low conductivity of sulfur and its discharge product (Li2S2/Li2S), the shuttle effect of lithium polysulfide (Li2Sn, 4 ≤ n ≤ 8) during charging/discharging process and so on. Here, cobalt disulfide/reduced graphene oxide (CoS2/rGO) composites were easily and efficiently prepared through an energy-saving microwave-assisted hydrothermal method and employed as functional interlayer on commercial polypropylene separator to enhance the electrochemical performance of Li-S battery. As a physical barrier and second current collector, the porous conductive rGO can relieve the shuttle effect of polysulfides and ensure fast electron/ion transfer. Polar CoS2 nanoparticles uniformly distributed on rGO provide strong chemical adsorption to capture polysulfides. Benefitting from the synergy of physical and chemical constraints on polysulfides, the Li-S battery with CoS2/rGO functional separator exhibits enhanced conversion kinetics and excellent electrochemical performance with a high cycling initial capacity of 1,122.3 mAh g-1 at 0.2 C, good rate capabilities with 583.9 mAh g-1 at 2 C, and long-term cycle stability (decay rate of 0.08% per cycle at 0.5 C). This work provides an efficient and energy/time-saving microwave hydrothermal method for the synthesis of functional materials in stable Li-S battery.

N, S-doped graphene derived from graphene oxide and thiourea-formaldehyde resin for high stability lithium-sulfur batteries

Although lithium-sulfur (Li-S) batteries with a high theoretical energy density and low cost have attracted extensive research attention, their commercialization is still unsuccessful due to the poor cycle life caused by the dissolution of polysulfides. It is the key challenge to overcome polysulfide shuttling for achieving long-term cycling stability in Li-S batteries. Here we report a novel strategy for the synthesis of N, S-doped graphene with high nitrogen and sulfur contents via in situ self-assembly of graphene oxide and thiourea-formaldehyde resin and calcination. The N, S-doped graphene serves as a conductive agent and a chemosorbent for suppressing polysulfide shuttling and preventing the Li-metal from corrosion, leading to a high reversible capacity and superior cycling stability. The Li-S batteries with the N, S-doped graphene can achieve an excellent cycling life (622 mA h g^-1 after 500 cycles at 1C) and a slow capacity decay rate (0.049% per cycle over 500 cycles at 1C). The proposed strategy has the potential to enhance the high electrochemical properties of Li-S batteries.

Understanding the modulation effect and surface chemistry in a heteroatom incorporated graphene-like matrix toward high-rate lithium-sulfur batteries

The underlying interface effects of sulfur hosts/polysulfides at the molecular level are of great significance to achieve advanced lithium-sulfur batteries. Herein, we systematically study the polysulfide-binding ability and the decomposition energy barrier of Li₂S enabled by different kinds of nitrogen (pyridinic N, pyrrolic N and graphitic N) and phosphorus (P-O, PO and graphitic P) doping and decipher their inherent modulation effect. The doping process helps in forming a graphene-like structure and increases the micropores/mesopores, which can expose more active sites to come into contact with polysulfides. First-principles calculations reveal that the PO possesses the highest binding energies with polysulfides due to the weakening of the chemical bonds. Besides, PO as a promoter is beneficial for the free diffusion of lithium ions, and the pyridinic N and pyrrolic N can greatly reduce the kinetic barrier and catalyze the polysulfide conversion. The synergetic effects of nitrogen and phosphorus as bifunctional active centers help in achieving an in situ adsorption-diffusion-conversion process of polysulfides. Benefiting from these features, the graphene-like network achieves superior rate capability (a high reversible capacity of 954 mA h g^-1 at 2C) and long-term stability (an ultralow degradation rate of 0.009% around 800 cycles at 5C). Even at a high sulfur loading of 5.6 mg cm^-2, the cell can deliver an areal capacity of 4.6 mA h cm^-2 at 0.2C.

Multifunctional Metal Phosphides as Superior Host Materials for Advanced Lithium-Sulfur Batteries

For the past few years, a new generation of energy storage systems with large theoretical specific capacity has been urgently needed because of the rapid development of society. Lithium-sulfur (Li-S) batteries are regarded as one of the most promising candidates for novel battery systems, since their resurgence at the end of the 20th century Li-S batteries have attracted ever more attention, attributed to their notably high theoretical energy density of 2600 W h kg^-1 , which is almost five times larger than that of commercial lithium-ion batteries (LIBs). One of the determining factors in Li-S batteries is how to design/prepare the sulfur cathode. For the sulfur host, the major technical challenge is avoiding the shuttling effect that is caused by soluble polysulfides during the reaction. In past decades, though the sulfur cathode has developed greatly, there are still some enormous challenges to be conquered, such as low utilization of S, rapid decay of capacity, and poor cycle life. This article spotlights the recent progress and foremost findings in improving the performance of Li-S batteries by employing multifunctional metal phosphides as host materials. The current state of development of the sulfur electrode of Li-S batteries is summarized by emphasizing the relationship between the essential properties of metal phosphide-based hybrid nanomaterials, the chemical reaction with lithium polysulfides and the latter's influence on electrochemical performance. Finally, trends in the development and practical application of Li-S batteries are also pointed out.

A multifunctional UiO-66@carbon interlayer as an efficacious suppressor of polysulfide shuttling for lithium-sulfur batteries

Restraining the shuttle effect in lithium-sulfur (Li-S) battery is crucial to realize its practical application. In this work, a UiO-66@carbon (UiO-66@CC) interlayer was developed for Li-S battery by growing a continuous UiO-66 film on carbon cloth. The continuous UiO-66 crystal layer contributes to provide sufficient adsorptive and catalytic sites for efficient adsorption and catalytic conversion towards polysulfides. Moreover, the hydrophilic property of UiO-66 material ensures the full infiltration of electrolyte and accelerates the transportation of lithium ions. Profiting from the above advantages of the proposed interlayer, the shuttle effect is effectively inhibited and a fast redox kinetic is also realized. Accordingly, the Li-S battery using UiO-66@CC delivers a specific capacity of 1228.9 mAh g^-1at 0.2 C with a nearly 100% capacity retention after 100 cycles, and the first specific capacity is 1033.1 mAh g^-1at 1.0 C with a decay rate of 0.07% over 600 cycles. Meanwhile, UiO-66@CC interlayer also has an excellent rate performance with a specific capacity of 535.9 mAh g^-1at 5.0 C and a high area capacity of 6.2 mAh cm^-2at increased sulfur loading (8.15 mg cm^-2).

Three-Dimensionally Ordered Macro/Mesoporous Nb2O5/Nb4N5 Heterostructure as Sulfur Host for High-Performance Lithium/Sulfur Batteries

The severe shuttle effect of soluble polysulfides hinders the development of lithium–sulfur batteries. Herein, we develop a three-dimensionally ordered macro/mesoporous (3DOM) Nb₂O₅/Nb₄N₅ heterostructure, which combines the strong adsorption of Nb₂O₅ and remarkable catalysis effect of Nb₄N₅ by the promotion “adsorption-transformation” mechanism in sulfur reaction. Furthermore, the high electrocatalytic activity of Nb₄N₅ facilitates ion/mass transfer during the charge/discharge process. As a result, cells with the S-Nb₂O₅/Nb₄N₅ electrode delivered outstanding cycling stability and higher discharge capacity than its counterparts. Our work demonstrates a new routine for the multifunctional sulfur host design, which offers great potential for commercial high-performance lithium–sulfur batteries.

Co9 S8 @CN Composites Obtained from Thiacalix[4]arene-Based Coordination Polymers for Supercapacitor Applications

Metal sulfides have been recognized as promising electrodes for electrochemical energy storage owing to their remarkable electrochemical properties. Here, we demonstrate the preparation of Co₉ S₈ nanoparticles anchored on a carbon matrix (denoted as Co₉ S₈ -X@CN (X=1, 2)) from precursor sources, two 1D infinite coordination polymers 1 and 2. The two polymers were assembled by linking Co₄ -TC4A secondary building blocks (SBUs) with ligands L₁ and L₂ , respectively (H₄ TC4A=p-tert-butylthiacalix[4]arene, L₁ =1,4-bis(2H-tetrazol-5-yl)benzene, L₂ =1,3-bis(2H-tetrazol-5-yl)benzene). The composites obtained from 1D polymers showed different morphologies, that is, the Co₉ S₈ nanoparticles of Co₉ S₈ -1@CN are octahedral with a size of ca. 140 nm, while the lamellar Co₉ S₈ nanoparticles in Co₉ S₈ -2@CN possess different sizes (50-150 nm). The Co₉ S₈ -2@CN immobilized on nickel foam (Co₉ S₈ -2@CN/NF) show better supercapacitive performance than that of Co₉ S₈ -1@CN. Co₉ S₈ -2@CN showed exceptionally high activities, combining higher specific capacitances (445.2 F g^-1 at 2 A g^-1 and 393.9 F g^-1 and 5 A g^-1 ), rate capacity (94.5% retention at 2 A g^-1 ), and long-term stability (79.2% retention at 5 A g^-1 over 1000 cycles). The smaller size and larger BET surface area of Co₉ S₈ -2@CN nanoparticles can improve the electrical conductivity and provide facile pathways for charge transport, thus leading to conspicuous electrochemical performance of Co₉ S₈ -2@CN compared with its Co₉ S₈ -1@CN counterpart.

A. R, Fouad OA. Polyvinylpyrrolidone and freeze drying-assisted growth of an α-Ni(OH)2/reduced graphene oxide hybrid structure as a superior electrode material for supercapacitors

The electrode of 30 wt% α-Ni(OH)₂/rGO hybrid structure displayed a promising material for supercapacitors. It showed a specific capacitance (capacity) of 1050 F g⁻¹ (580 C g⁻¹), and its hybrid device exhibited a high energy density of 60 W h kg⁻¹.